The invention relates to a method of continuously casting steel strands, in particular slabs, in a continuous casting plant, wherein the strand is withdrawn from a cooled oscillating mold containing molten steel covered by a casting slag layer or a casting powder layer, and wherein said strand is further cooled in a secondary cooling zone arranged to follow the mold.
With the oscillation of the mold, so-called oscillation marks occur on the surface of the withdrawn strand. These oscillation marks have an adverse effect on the strand surface with the known method of operation. When the strand is being cooled in the secondary cooling zone it runs through a temperature range in which it is deformable only to a minor degree. This range is called the red shortness range and in killed, unalloyed steels it lies between approx. 800.degree.C and 950.degree.C. In this range the strand is liable to crack formation, in particular at its edges and in an area close to said edges. The oscillation marks on the strand surface constitute points of discontinuity, both as regards the surface condition and as regards the temperature. The marks are formed by strand regions in which the slag particles, carried along out of the mold, become deposited to an increased degree as compared to the rest of the surface and in which scale settles. The result is that these points are cooled less by the cooling water sprayed onto the strand. Moreover, these temperature discontinuities have the consequence that expansions which occur in the cast strand become effective almost exclusively in the oscillation marks because these points have, on account of their higher temperature, a lower resistance to deformation. Thus the mechanical stresss exerted upon the strand when said strand is being withdrawn from the secondary cooling zone is not uniform. Whereas the strand surface hardly increases in length between the oscillation marks, the oscillation marks themselves are extended and overextended. As a consequence thereof fine surface cracks occur which, although not visible with the naked eye, constitute dangerous points of weakness, which, on account of tension concentrations, easily result in severe surface defects, i.e. in cracks having a depth of a few mm. These cracks then have to be removed by scarfing, which entails a loss of material and high costs.